In many applications, the qualities of lightweight construction and high resistance to fragmentation and corrosion damage are highly desirable characteristics for a pressure vessel. These design criteria have been met for many years by the development of high pressure composite (fiber reinforced resin matrix) containers; for instance, containers fabricated of laminated layers of wound fiberglass filaments or various types of other synthetic filaments which are bonded together by a thermal-setting or thermoplastic resin. An elastomeric or other non-metal resilient liner or bladder often is disposed within the composite shell to seal the vessel and prevent internal fluids from contacting the composite material.
Such composite vessels have become commonly used for containing a variety of fluids under pressure, such as storing oxygen, natural gas, nitrogen, rocket or other fuel, propane, etc. The composite construction of the vessels provides numerous advantages such as lightness in weight and resistance to corrosion, fatigue and catastrophic failure. These attributes are due to the high specific strengths of the reinforcing fibers or filaments which typically are oriented in the direction of the principal forces in the construction of the pressure vessels.
Composite pressure vessels of the character described above originally were developed for aircraft and aerospace applications primarily because of the critical weight restrictions in such vehicles. These applications provided a relatively safe environment in which damage to the vessels could be minimized and, in fact, impact damage from extraneous, unintended collisions rarely occurred. However, the growing use of composite pressure vessels in general commercial applications has significantly increased the potential for the vessels to be subjected to uncontrolled damage which may significantly affect the strength of a vessel without showing any obvious visual damage. For instance, during shipment or other handling, a vessel may be dropped and suffer interior or structural damage which is visually undetectable when observing the exterior or shell of the vessel. A damaged vessel might be installed for its intended or ultimate use without anyone even knowing that the vessel was damaged.
Some contemporary approaches to solving these problems have included increasing the shell or wall thicknesses of the vessels, using sacrificial material on the exterior surfaces of the vessels and applying rubber or other elastomer coatings to the vessels. Such systems actually involve adding some sort of protective feature to the surface of the vessels after the vessels have been primarily constructed. They function more to prevent damage to the vessels rather than provide visual evidence that damage may have occurred. In addition, these expedients which involve adding extraneous materials to the outside of the vessels can and do increase the overall size and weight of the vessels. Increasing the composite wall thickness of a vessel to prevent damage thereto simply defeats the purpose of providing a lightweight structure. Adding sacrificial material, such as a layer of fiberglass over an entire vessel so that the layer is cut, gouged or punctured without changing the integrity of the composite shell of the tank, again simply is adding an additional thickness to the vessel itself. The same disadvantages apply to the use of rubber or other elastomer coatings to a vessel, and such coatings are significantly heavier than the same thickness of a composite material. All of these expedients also have the disadvantage of potentially obscuring the damage which they are intended to prevent, just contrary to the concepts of the present invention as disclosed and claimed herein. In other words, a damage-preventing external coating or cover that does not sustain visually obvious surface damage provides no evidence to an inspector that a damage-inducing event has occurred, even though structural damage may have been sustained by the primary composite structure beneath the area of impact.
The present invention is directed to solving the above problems and mitigating the results of impact damage by making serious damage easy to visually detect while not changing the appearance of the vessel in any other respect.